Ten Year Old Supernova Mystery Solved

A small team of European astronomers together with University of Hawaii Institute for Astronomy Director Rolf Kudritzki has found the first observational evidence that a famous supernova came from a binary system of stars orbiting around each other. Supernova explosions are spectacular cosmic events, during which stars suddenly flash with an enormous brightness so that they can still be seen at large cosmic distances and can outshine all the other billions of stars in their host galaxy. Supernova explosions are also some of the most important "producers" of chemical elements in the Universe, like the oxygen we all breathe, so it is essential that we know what type of stars produce them. "Life in the universe and on earth would not exist without supernovae. We all consist of 'stardust' which was created through supernovae explosions", says Dr. Rolf Kudritzki.

The second brightest supernova discovered by humans in modern times, SN 1993J, was found in the beautiful spiral Galaxy M81 on March 28 1993. It first appeared rather ordinary but soon began to puzzle astronomers as instead of fading — as other supernovae do normally — it showed a bizarre sharp increase in its brightness. In addition, as the spectroscopic analysis of its light showed, the ejecta from the explosion seemed far too rich in the chemical element helium.

From archival images of this galaxy taken before the explosion, a gigantic red supergiant star was identified as the progenitor. In the history of astronomy this was only the second time that astronomers have seen the progenitor, or mother star, of a supernova explosion (the first was SN 1987A, the supernova that exploded in 1987 in our neighbor galaxy, the Large Magellanic Cloud). But a normal red supergiant alone could not have given rise to such a weird supernova, and astronomers suggested the red supergiant orbited a companion which had shredded its outer layers just before explosion.

Using the giant Keck telescope on Mauna Kea in Hawaii and the Hubble Space Telescope the astronomers have now peered deep into the 10 year old, but still glowing supernova remnant. Exactly at the position of the supernova they have discovered a massive star which has got to be a companion to the supernova progenitor. This proves that this strange supernova did indeed occur in a double star system. It also allows a detailed investigation of the stellar physics leading to supernovae explosions. By observing the companion closely in the coming years it may be possible to detect a neutron star or black hole emerge from the remnants of the explosion "live".

According to Rolf Kudritzki, "The combination of the huge light-gathering power of the Keck 10-m telescope in Hawaii and the outstanding spatial resolution of Hubble has made this unique discovery possible".

This discovery provides the first direct observational evidence that peculiar supernovae of this type are due to very massive stars in binary systems. It allows to determine very precisely the physical properties of the two stars in the binary system, before one exploded and the remainder was substantially changed as the result of the explosion. Given the paucity of observations of supernova progenitors this result published in the famous international journal NATURE on January 8, 2004 is likely to "be crucial to understanding how very massive stars explode and why we see such peculiar supernovae" according to first author Justyn R. Maund from University of Cambridge.

When a star of more than about 8 times the mass of the sun reaches the end of its nuclear fuel reserve, it can no longer produce enough energy to keep it from collapsing under its own immense weight. The core of the star collapses, and the outer layers are ejected in a fast moving shock wave. This huge energy release causes the visible supernova we see. While astronomers are confident that their theories can match this model, they are in the embarrassing position that they have confidently identified only two stars that later exploded as supernovae.

There have been more than 2,000 supernovae discovered in galaxies beyond the Milky Way and there appear to be about eight distinct classes. However identifying which stars produce which flavors has proved incredibly difficult. In its collaboration the team has embarked on a parallel project with the Hubble Space Telescope to image a large number of galaxies and wait patiently for a supernova to explode. Supernovae appear in spiral galaxies like M81 on average once per 100 years or so and the team led by Stephen Smartt from the University of Cambridge hopes to increase the numbers of supernovae progenitors known from 2 to 20 over the next five years.

Stephen Smartt (University of Cambridge) says "Supernovae explosions are at the heart of our understanding of the evolution of Galaxies and the formation of the chemical elements in the Universe. It is essential that we know what type of stars produce them." For the last ten years astronomers have believed that they could understand the very peculiar behavior of 1993J by invoking the existence of a binary companion star and now this picture has been proved correct. "Sometimes in science it pays off to be patient", says UH IfA Director Rolf Kudritzki, "but it took the power of the Keck telescope and the Hubble to find the spectroscopic fingerprints of the companion and to track it down".

The Institute for Astronomy at the University of Hawaii conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea. Refer to http://www.ifa.hawaii.edu/for more information about the Institute.

A virtual travel into one of the spiral arms of grand spiral Messier 81 reveals the superb razor-sharp vision of the NASA/ESA Hubble Space Telescope. The close-up (to the right) is centred of the just discovered companion star to Supernova 1993J which no longer is visible. The quarter-circle around the supernova companion is a so-called light echo which comes from sheets of dust in the galaxy reflecting light from the original supernova explosion.

This NASA/ESA Hubble Space Telescope image shows the area in Messier 81 where Supernova 1993J exploded. The companion to the supernova "mother star" that remains after the explosion is seen in the centre of the image.

The image is taken with Hubble's Advanced Camera for Surveys and is a combination of four exposures with ACS's High Resolution Camera. The exposures were taken through a two near-UV filters (250W, 2100 seconds and 330W, 1200 seconds) shown in purple and blue, a deep blue filter (435W, 1000 seconds) shown in green and a green filter (555W, 1120 seconds) shown in red.

The quarter-circle around the supernova companion is a so-called light echo which comes from sheets of dust in the galaxy reflecting light from the original supernova explosion. The timing of the appearance of these echoes can be used to map out the dust structure around the supernova.

Credit: ESA, NASA and Justyn R. Maund (University of Cambridge)

3. heic0313c: Messier 81 spiral arm (WFPC2 image)

This NASA/ESA Hubble Space Telescope image shows a small portion of one of Messier 81's spiral arms. It extends about 0.03 x 0.03 degrees. The supernova companion is the bluish star in the upper right hand corner. Dust lanes in the spiral arms of the galaxy are seen, as well as many other stars and a few nebulae.

The image is composed of four separate exposures from the ESO/ST-ECF Archive through a blue filter, a green filter, a red filter and a near-infrared filter.

Credit: ESA, NASA and Justyn R. Maund (University of Cambridge)

Acknowledgment: Bob Kirshner (Harvard University, USA)

4. heic0313d: Grand Spiral Messier 81 (ground-based)

This ground-based image shows the spiral galaxy Messier 81 in its entirety. The image is a combination of exposures from the Isaac Newton Telescope on La Palma (courtesy of Jonathan Irwin) and Digitized Sky Survey 2 images.

Credit: ESA/INT/DSS2

5. heic0313e: The dynamic duo, Messier 81 and 82 (ground-based)

In this wide-angle image taken by astrophotographer Robert Gendler the amazing duo of Messier 81 (left) and Messier 82 (right) are seen. These two mighty galaxies in the Big Dipper (Ursa Major) belong to some of the most famous and beloved galaxies to amateur astronomers. This may be one of the reasons that Supernova 1993J was discovered by the Spanish amateur astronomer Francisco Garcia Diaz.

Credit: Robert Gendler (http://www.robgendlerastropics.com/)

6. heic0313f: Constellations Ursa Major and Ursa Minor

This image was taken by astrophotographer Akira Fujii and shows the Big Dipper (Ursa Major) in the lower right, and Ursa Minor to the left of centre.

Credit: Akira Fujii

Links:

The scientific paper: http://www.nature.com

Paper 1 on the light echo:http://esoads.eso.org/cgi-bin/nph-data_query?bibcode=2002ApJ...581L..97S&db_key=AST&link_type=ABSTRACT&high=3fa7b7bf2c25549